This invention relates in general to the oxonation or hydroformylation of unsaturated cyclic ethers and in particular to the hydroformylation of dihydrofuran derivatives.
Hydroformylation of olefins has been known since 1938 and was commercialized in the early 1950's as a method of producing alcohols by reducing the aldehydes formed through the hydroformylation reaction. In this reaction hydrogen and carbon monoxide are added across an olefinic bond to produce aldehydes containing one more carbon atom than the olefin.
In the hydroformylation reaction, catalysis of the reactants can be homogeneously carried out through carbonyls of group VIII metals, such as iron, cobalt, nickel, ruthenium, rhodium, palladium, osmium, iridium & platinum. Although the exact hydroformylation reaction mechanism is not completely understood, it is now generally agreed that it involves alkyl and acyl metal carbonyls as intermediates. Further, it is reasonably well established that "carbonyl insertion", the migration of a carbon monoxide ligand in the carbonyl to a position between the alkyl group and the metal atom takes place.
In spite of the great deal of study and the commercial success of the hydroformylation reactions with olefins for forming aldehydes and subsequently alcohols, the preparation of aldehydes in good yield from the hydroformylation of unsaturated cyclic ethers such dihydrofurans has not been reported.
Furans have reportedly been hydroformylated to hydroxymethyl tetrahydrofurans in modest yield using Co.sub.2 (CO).sub.8 and dihydropyrans have been hydroformylated using Co.sub.2 (CO).sub.8 to hydroxymethyl tetrahydropyrans in good yield. However, in either case only low yields of the intermediate aldehydes could be isolated. In the hydroformylation reaction, rhodium complexes (e.g., Rh H (CO) (PPh.sub.3).sub.3 and Rh Cl (CO) (PPh.sub.3).sub.2) have been found to be more active hydroformylation catalysts than cobalt compounds.